So far, photochemical reactions are generally carried out by a batch production system in which a batch of a reaction mixture such as, for example, a mixture of vinyl monomers and additives is put into a reaction vessel and then irradiated with ultraviolet light from a plurality of ultraviolet lamps arranged in the vessel, while being stirred by a stirrer coaxially arranged in the center of the vessel.
Since the rate of photochemical reaction depends on the intensity of ultraviolet light, the reaction of monomers is enhanced in an area close to the outer walls of the lamps and the reaction mixture is increased in viscosity of with increase of polymerization degree. the viscosity of the reaction product may vary within the range of from 1000 to 150,000 cps. If the viscosity is increased to 100,000 cps and above, the stirring of the reaction mixture takes place only around the stirrer, resulting in a local photopolymerization, which in turn results in production of polymers with non-uniform molecular weights. This problem becomes large with increase in the size of the reaction vessel.